1. Technical Field
The present invention relates to an imaging lens that forms an optical image of an object to be shot on an image pickup element such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) and an image pickup device such as a digital still camera, a portable cellular phone having a camera and an information portable terminal (PDA: Personal Digital Assistance on which the imaging lens is mounted to shoot the object to be shot.
2. Related Art
In recent years, with the spread of personal computers in ordinary homes, digital still cameras are rapidly popularized to which image information such as the images of shot landscapes or persons can be inputted. Further, camera modules for inputting images are frequently mounted on portable cellular phones. In device having such an image pickup function, the image pickup elements such as the CCD or the CMOS are used. In these years, a miniaturization of the image pickup elements progresses, so that the miniaturization of the entire part of an image pickup devices and the imaging lenses mounted thereon is required. Further, at the same time, pixels of the image pick-up elements are progressively increased and the high resolution and high performance of the imaging lenses are required. For instance, a performance satisfying high pixels such as 2 mega pixels or higher or more preferably, 5 mega pixels or higher is requested.
To satisfy above-described requests, the imaging lens of a structure using four lenses as a whole has been developed (see patent document 1 (JP-A-2005-4028 corresponding to US-A-2004-228009) and patent document 2 (JP-A-2005-208236)).
However, in an imaging lens described in the above-described patent documents, since a second lens has a concave shape directed to an object side, an entire length is increased so that a miniaturization is difficult. Further, since many field curvatures are generated, a problem arises that a high image quality is hardly obtained. Further, in the imaging lens including four lenses having an F number of 2.8 or so, the power of a first lens is usually apt to be relatively strong in a lens system to decrease the entire length. Therefore, the performances of the field curvatures are liable to be deteriorated. While the imaging lens is requested to have a faster (the F number of about 2.4) lens system, the imaging lens is occasionally requested to shoot an object under a state that a fastness is restricted depending on a shooting environment. However, in a compact image pickup device, an aperture diaphragm is frequently a fixed diaphragm in view of a mechanical problem and a problem of cost. In this case, to obtain a state that the fastness is restricted, for instance, a method is considered in which the fast lens system of the F number of about 2.4 is ordinarily used and a small aperture diaphragm having a small diameter is detachably arranged at the nearest position to the object side of the lens system as required to change the F number. Thus, a dynamic range can be enlarged during a shooting operation and a necessary depth of a focal point can be ensured. However, under the state that such a small aperture diaphragm is arranged, since the position of the aperture diaphragm is changed relative to an ordinary state, an image formation performance is changed. Accordingly, when an optical design that considers only an ordinary fast state is drawn, the image formation performance is liable to be deteriorated (especially, the field curvatures are deteriorated) under the state that the small aperture diaphragm is arranged, so that the lens system is insufficient in its performance. Therefore, a lens is desired to be developed that has a performance meeting high pixels, for instance, 2 mega pixels or higher, and more preferably 5 mega pixels to 10 mega pixels or so.
The present invention is devised by considering the above-described problems, and it is an object of the present invention to provide an imaging lens that maintains a good image formation performance, is compact and fast and can realized a lens system whose performance is hardly deteriorated even when the position of an aperture diaphragm is changed and an image pickup device that can obtain a picked-up image of high resolution by mounting the imaging lens thereon.